This invention relates to papermaking and more particularly to improvements in a belt for a shoe press, the belt being designed to be introduced into the high-temperature nip of a papermaking machine to effectively squeeze water out of the wet web in the press section of the machine.
For high temperature papermaking technology, a technique known as impulse drying has recently been proposed, wherein water is squeezed out of a fibrous web by pressing the web by means of a shoe press at a high temperature, typically 200xc2x0 C. or greater, and ranging even up to 350xc2x0 C. By this process, dewatering is accomplished not only by squeezing water out of the fibrous web under pressure applied by the press, but also by evaporation of water from inside the fibrous web as a result of heating. The web is exposed to a high temperature over the relatively long duration of its passage through the press part of the machine.
Although we do not wish to be bound by any particular theory, we believe that the heating of the fibrous web may cause the water contained in the web to decrease in viscosity, and thereby permit more efficient squeezing than achieved at conventional press temperatures. High temperature, impulse drying technology is discussed in Japanese Patent Publications 63195/1994 and 33590/1994, in Japanese Patents 2590170 and 2832713, and in Published International application WO97/15718 (laid-open in Japan under PCT Application number 500793/1999).
Japanese Patent Publication 33590/1994 is concerned particularly with a belt which has grooves in its surface to ensure efficient squeezing of water while preventing paper breakage and poor formation, the breakage being due to the removal of a large amount of water from the fibrous web during pressing at high temperature and high pressure.
A problem presented by the technology briefly described above is that the belt is constructed of a resin layer which is subject to softening when exposed to high temperature and high pressure. The softened resin layer tends to deform, clogging the grooves and thereby reducing the amount of water removed. Moreover, the softened resin layer wears readily, thereby both decreasing the volume of the grooves and shortening the service life of the belt.
In addition, when the belt is operated at high temperature, its durability is impaired by thermal degradation of its constituents, i.e., both the supporting fabric and the resin. The degradation of the supporting fabric leads to dimensional instability, and the degradation of the resin leads to cracking of the belt.
Another problem encountered in the use of a shoe press belt at high temperature is that the lubricant used to reduce friction between the belt and the shoe decreases in viscosity with increasing belt temperature. The decrease in viscosity of the lubricant results in an increase in the driving load on the papermaking machine.
We have undertaken intensive investigations of the aforementioned problems with the objective of providing a shoe press belt so constructed as to isolate or protect its resin layer from external heat so that the resin layer does not soften and its grooves do not deform during operation at high temperature and high pressure.
The improved shoe press belt in accordance with our invention consists of a base layer and a resin layer, the resin layer having a surface facing the base layer and an opposite surface with a groove for promoting dewatering. The shoe press belt is characterized by the fact that both the base layer and the resin layer are made from a heat-resistant material and the resin layer contains a filler to control its thermal conductivity. Thus, the belt is constructed in such a way that both the base layer and the resin layer have improved heat resistance, and the belt is less subject to the effects of external heat.
In accordance with one embodiment of the invention, the filler is composed of a material having a thermal conductivity lower than that of the material of the resin layer, to prevent the temperature of the resin layer from increasing excessively due to external heat.
In accordance with another embodiment of the invention, the filler is composed of a material having a thermal conductivity higher than that of the material of the resin layer, so that the resin layer more effectively expels heat which enters the resin layer from outside, thereby cooling itself more rapidly.
In accordance with still another embodiment of the invention, the resin layer is composed of a plurality of sublayers placed one over another, and at least one, but preferably not all, of said sublayers contains the filler. Constructed in this way, the belt can have its thermal conductivity properly controlled without affecting the performance of the resin at the surface.
In still another embodiment in which the resin layer is composed of a plurality of sublayers placed over one another, each of at least two sublayers contains a filler, and the thermal conductivity of each sublayer containing a filler differs from the thermal conductivity of each of the other sublayers. Each of the sublayers may contain a filler, or, alternatively, some, but not all, of the sublayers may contain a filler. The thermal conductivity of the several layers may be controlled by utilizing fillers having different thermal conductivities, or alternatively, by incorporating different concentrations of filler in the different layers. In this embodiment, the belt can have a changing thermal conductivity throughout the thickness of its resin layer, either from low to high, or from high to low, for efficient heat control. With a sufficient number of layers, the change in thermal conductivity can be made effectively continuous.
As will be apparent from the following description, advantage can be taken of various combinations of the above-described features to achieve control of heat and to prevent the adverse effects of excessive heat in a shoe press belt operated at high temperature and high pressure.
Further objects, details and advantages of the invention will be apparent from the following detailed description, when read in conjunction with the drawings.